Generally, a wireless local area network (WLAN) is a typical wireless network. Unlike the conventional wired LAN, the WLAN uses a wireless medium as a medium for a data transmission, without the necessity of connection of a cable used for the conventional wired LAN. Currently, the WLAN transmits and receives data by using radio resources, and as such, the WLAN receives much attention to extend or substitute the conventional wired LAN.
The WLAN is based on the IEEE 802.11 standards of the IEEE (Institute of Electrical and Electronics Engineers). When multicast is applied to an MSDU (Medium Access Control Service Data Unit), it is an MSDU having a multicast address as a receiver address (RA) or a destination address (DA). When the multicast address is applied to an MPDU (Medium Access Control Protocol Data Unit) or a control frame, it is an MPDU or a control frame having a multicast address as an RA. Here, the multicast address refers to a MAC (Medium Access Control) address having a group bit set. The MSDU refers to information transferred to a unit between MAC service access points. The MPDU refers to a unit of data exchanged between two equal MAC layers by using a service of a physical layer.
In the WLAN, data is transmitted by using a DCF (Distributed Coordination Function), and the DCF, a basic medium access scheme of IEEE 802.11, is to share a medium between terminals by using a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol and a random backoff time. Transmission of every unicast frame is checked through acknowledgement (ACK) signal, and if there is no response within an ACK signal standby time, retransmission is attempted. If the ACK signal is not received until even after retransmission is attempted several times (e.g., five to 21 times), the unicast frame is discarded. This means a transmission failure.
However, transmission of a broadcast frame and a multicast frame is not checked by using such an ACK signal. Namely, the MAC of IEEE 802.11 standards basically supports the unicast frame but not the broadcast frame nor a multicast frame.
As a result, transmission of the broadcast frame and multicast frame has a high probability of a transmission failure due to interference of adjacent channels, multi-path fading, or the like, and because there is no retransmission, one time of failure makes the frame discarded. This means a failure of radio (wireless) transmission of the multicast frame. That is the characteristics of the radio (wireless) environment contrasted with a fixed line environment, in which transmission of multicast frames can hardly guarantee a stable reception due to the transmission failure, and QoS (Quality of Service) is irregular according to a channel state. Thus, the related art radio transmission of multicast frames has a high possibility of a failure and degrades QoS.